1. Field of the Invention
The present invention relates to a novel torque sensor device and more particularly to a novel torque sensor device which detects the torque applied to a rotating shaft and converts the detected torque into an electrical signal.
2. Description of the Prior Art
It is well known that various magnetic properties of magnetic materials change due to applied stress. In particular, the permeability of the magnetic material tends to increase due to tensile stress and tends to decrease due to compressive stress. This is mathematically expressed as follows: EQU .mu.=K.sub.1 +K.sub.2 .sigma. (1)
where:
.mu.=permeability PA1 K.sub.1 =constant PA1 K.sub.2 =constant PA1 .sigma.=applied stress PA1 N=number of turns in coil PA1 a=radius of coil PA1 l=length of coil
In the unstressed condition .sigma. is equal to zero and thus .mu. is equal to the constant K.sub.1 which is the normal unstressed permeability of the magnetic material. When a stress .sigma. is applied, the permeability is changed from K.sub.1 by a factor K.sub.2 .sigma.. Thus, the permeability of the magnetic material is directly proportional to the applied stress.
When a torsional stress is applied to a cylindrical rod, each element in the rod is subjected ro a shearing stress. BETH and MEEKS in their article "Magnetic Measurement Of Torque In A Rotating Shaft" (The Review of Scientific Instruments, Vol. 25, no. 6, June, 1954) describe an analysis of the effects of torsional stress in a magnetic cylinder. It was determined that the shearing stress applied to any single element in the cylinder could be expressed in terms of a tensile stress and an equal perpendicular compressive stress. The magnitude of each stress is directly proportional to the distance between the axis of the cylinder and the single elment in question. Further, it was determined that, when considering the cylinder as a whole, the directions of maximum tension and compression occur along 45.degree. left and 45.degree. right-handed helices about the axis of the cylinder as illustrated in FIG. 1 wherein a cylindrical rod 1 is shown as being subjected to a torque M. Helical line 2 represents the direction of compressive stress occurring along the surface of the cylindrical rod while helical line 3 represents the direction of tensile stress. Reference numeral 4 represents a typical element of the cylindrical rod 1. Element 4 is thus clearly subjected to a compressive stress along helical line 2 and a tensile stress along helical line 3; helical lines 2 and 3 being mutually perpendicular. In a cylindrical rod made of magnetic material, the effect of the torque M is to increase the magnetic permeability in directions parallel to helical line 3 and correspondingly to decrease the magnetic permeability in directions parallel to helical line 2.
Prior art torque sensors have utilized the above principles to determine changes in the permeability of a magnetic rod or drive shaft subjected to torque. This has been accomplished in one such prior art device by applying a magnetic field to the magnetic rod and detecting changes, due to torque applied to the rod, in the induced EMF occurring in various pick-up coils located adjacent to the rod.
While such prior art devices are capable of determining the amount of applied torque, they are not without significant drawbacks. For example, in a practical application where the magnetic rod acts as a drive shaft, in order to withstand the mechanical effects of the applied torque, the magnetic rod must be made from physically strong materials which may not be desirable from a magnetic standpoint. Further, typical forged drive shafts are inherently anisotropic due to inherent stresses and crystal orientations occurring within the shaft. This results in variations in the magnetic permeability of the drive shaft at various points in the shaft. Since the stress due to applied torque varies directly with distance from the axis of the drive shaft, magnetic sensors placed adjacent to the shaft are influenced both by the effects of anisotropy along the surface and in the interior of the shaft thereby providing many sources of essentially unpredictable error. Also, since each element in the drive shaft is simultaneously subjected to a tensile stress and a mutually perpendicular compressive stress, the magnetic sensors must be carefully positioned so as to be able to distinguish between the effects of both types of stress.
The present invention provides an electromagnetic torque sensor which overcomes these and other problems associated with prior art devices.